Description of the Prior Art
The prior foreign art describes a fire alarm signal box with a covering and the electrode adjacent thereto constituted by grids which are fastened to the outside edges of a tubular housing wall and between which a blower is arranged. This blower conveys ambient air through the covering and the electrode adjacent thereto, and along a plate electrode further distant from the covering and impermeable to air, to the open end, opposite to the covering, of the tubular housing wall, which end is covered by a further protective grid. Since ions are torn away by the air stream from the area between the two electrodes, a further blocking electrode, formed by a grid, is provided in the air stream, downstream of the plate electrode impermeable to air, which blocking electrode prevents such ions from leaving the ionization chamber and, for this purpose, is electrically connected to the electrode adjacent to the covering. The blower and the blocking electrode require a large structural expenditure.
An ionization fire alarm signal box is known wherein the ionization chamber is formed in the suction chamber of a cylinder in which a piston operating as an air conveyor moves back and forth. In that structure, the suction chamber is covered at opposite sides by a covering constituted by a filter, and open toward the cylinder. Two electrodes are provided in each case shaped as plate electrode impermeable to air, and arranged at a distance from one of the coverings that is small in comparison with the dimensions measured in the plate plane, and at a universal distance of their edge from the wall of the suction chamber. This structure presents the disadvantage that an air conveyor must be provided since otherwise the filters provided as coverings would strongly impede from entering the ambient air with particles carried by the fire.
In the aforementioned fire alarm signal boxes, the flow conditions in the ionization chamber are practically exclusively determined by flows produced by an air conveyor. Fire alarm signal boxes of a type similar to these are also known which operate without an air conveyor. In such fire alarm signal boxes, the difficulty occurs that flows of the ambient air lead to an undesirable change in sensitivity and to false alarms, and that this effect may vary according to the direction from which the flow approaches. For the elimination of this difficulty, various solutions are known.
An ionization fire alarm signal box is known which provides a wind guard in the form of cuplike protective shields, fitted into each other, with a multiplicity of small staggered openings, whereby the ambient air is repeatedly strongly deflected and thus strongly slowed down when entering the ionization chamber. The wind guard, however, likewise impedes the entry of ambient air mixed with combustion gases into the ionization chamber so that at low velocities of flow of the ambient air the sensitivity of the signal box is reduced.
The sensitivity to air flows is particularly marked in such ionization fire alarm signal boxes which have housings of small sizes since such housings can exert a function which protects the ionization chamber against air flows only to a small extent.
While the spaces normally and advantageously available for locating fire detectors, for instance in industrial plants, are of small sizes and it would also be logical to have the fire detectors of minute sizes in order to permit keeping them hidden from pranksters, mischiefmakers and saboteurs, the prior art neglected to consider these aspects.